1. Field of the Invention
The present invention generally relates to charged particle beam projection lithography exposure tools and, more particularly, to projection reticles used therein.
2. Description of the Prior Art
Lithographic processes are utilized in the manufacture of many diverse types of devices, particularly when very small areas must be selectively defined and/or operated upon, as in semiconductor integrated circuit manufacture. At least one lithographic process is invariably required for initial definition of locations and basic dimensions of devices such as transistors and capacitors in integrated circuits.
Lithographic processes currently used for integrated circuit manufacture involve the selective exposure of areas of a resist coated on a surface. In general, depending on whether the resist is of a positive or negative type, subsequent development will selectively remove either the exposed or unexposed areas leaving other areas substantially unaffected. In the past, radiant energy has been the resist exposure medium of choice. However, modern integrated circuit designs require feature sizes smaller than can be resolved using even very short wavelengths of light in the deep ultra-violet range even using sophisticated devices such as phase shift masks, off-axis illumination or optical proximity correction. Exposure of the resist with charged particle beams is required to obtain smaller feature sizes which are becoming increasingly common in current integrated circuit designs. Electron beams are generally preferred for charged particle beam exposures since, among other relative advantages, electrons allow control of the beam with both electric and magnetic means.
So-called probe-forming systems form a well-focussed spot at the target surface for exposure of the resist. xe2x80x9cGaussian beamxe2x80x9d systems, as the name implies, use a spot of Gaussian cross-section and either vector-address or raster-scan the beam to directly write the circuit of interest. Alternatively, shaped-beam systems, in particular, variable shaped beam (VSB) systems have higher throughput which is accomplished by parallel pixel exposure. A square shaping aperture is uniformly illuminated and imaged to another aperture, the size of which matches the image of the shaping aperture. The image of the shaping aperture is deflected onto the lower aperture and the compound image is then projected to the target (e.g. wafer). The Gaussian systems project one pixel at a time while the shaped beam systems can expose many pixels in parallel although the number of contiguous pixels concurrently exposed is relatively small.
For example, consider a rectangle of dimensions 0.1xc3x972.0 micrometers. Using a Gaussian beam with a 0.05 micrometer feature size, the rectangle corresponds to forty pixels. A shaped beam system with a maximum spot size of 1.0 micrometer square can expose this rectangle in two exposures.
In general, a single exposure for a shaped-beam system is limited to a few hundred pixels, at most, while the full pattern required for a full integrated circuit may include hundreds of millions of pixels or more. Therefore, the throughput of probe-forming exposure tools, even of the shaped-beam type, is too low to be economically feasible for high density, large scale integrated circuits even though exposures can be made at relatively high rate.
To obtain acceptable levels of throughput, electron beam projection lithography has been recently developed. Projection lithography projects a pattern (which may contain several millions of pixels) within a sub-field on a mask or reticle, containing a fraction of the complete circuit pattern, onto the target. This fraction may be small compared to a full pattern but is large compared to the dimensions of the beam in a probe-forming system. The pattern can be demagnified by the charged particle optics of the tool so that the pattern at the target is much smaller than the subfield pattern formed in the reticle.
The demagnified images of the reticle subfields formed from the beam pattern passing through or being scattered from the reticle in sequential exposures are suitably stitched together at the target or wafer to form the overall circuit pattern of the complete integrated circuit design. Generally, it is desirable that all subfield patterns that will be required for a given integrated circuit design be present on a common reticle. However, in the case of stencil reticles, complementary subfields are used to solve the xe2x80x9cdoughnutxe2x80x9d problem of printing closed features; respective parts of which must be exposed from different patterns since annular patterns cannot be fabricated as a stencil.
The practical requirement of providing all necessary patterns on a reticle, however, presents a problem in that even a very minor change in the design or layout of the integrated circuit requires a new reticle to be made incorporating the change. Reticles must be of extremely high precision and complexity as well as being defect-free and are thus extremely expensive to fabricate. Thus, changes and improvements in existing integrated circuit designs may substantially increase the overall cost of manufacture over the economic lifetime of the design.
Further, the so-called local Coulomb effect (LCE) may be particularly pronounced in electron beam projection lithography tools at large beam currents when exposing reticle subfields that contain both sparse and dense features in the same subfield. Local Coulomb effects are caused by the mutual repulsion of particles having the same charge and are manifested as both localized defocussing and therefore an increase in image blur where the exposure pattern is relatively more or less dense, and distortion of the pattern due to dislocation of the subfield features in regions where the density of the exposure pattern changes. These blurs and distortions may or may not be tolerable in a particular design or portion thereof.
These problems are major obstacles to the successful practice of charged particle beam projection lithography since they present limitations on economic feasibility and fidelity of imaging and generally impose trade-offs therebetween. Further, these problems are particularly intractable since they derive from unavoidable physical effects or force a trade-off between tool costs and tool throughput, which also forms a substantial economic cost component of the manufacturing process.
There is, accordingly, a need to extend the utility of charged particle beam projection lithography and electron beam projection lithography, in particular, by providing for direct writing of variable and controllable image shapes other than those directly defined by a particular reticle. Further, there is a need to provide for avoidance of local Coulomb effects, at least for pattern portions in which distortion and resolution may be particularly critical.
It is therefore an object of the present invention to provide a charged particle beam projection tool having the capability of beam shaping in a controllable manner.
It is a further object of the invention to provide a charged particle beam tools capable of making exposures which do not exhibit local Coulomb effect.
It is yet another object of the invention to provide a charged particle beam projection lithography tool capable of increased resolution and improved economy of operation.
In order to accomplish these and other objects of the invention, a charged particle beam projection lithography tool is provided comprising a shaping aperture, a patterned reticle having an open area, and an arrangement for imaging the shaping aperture at a portion of the open area of the reticle which includes a deflection arrangement for deflecting the shaped charged particle beam having a deflection center at a location of a source crossover of the charged particle beam projection lithography tool.
In accordance with another aspect of the invention, a method of charged particle beam projection lithography and semiconductor device manufacture is provided including steps of projecting a pattern formed in a patterned area of a reticle to a target, and imaging a shaping aperture onto a portion of an open area of the reticle to produce a compound image at said target.